1. Field of the Invention
The present invention relates to a micro-actuator for fine movement of a magnetic head and to a method of producing the same.
2. Description of the Related Art
In recent years, reductions in size and thickness of magnetic disk devices, a kind of external storage device for computers, have been under way and, further, reduction in electric power consumption is requested. In addition, magnetic disk devices with higher recording density and larger capacity are demanded. Larger capacity of the magnetic disk devices can generally be realized by increasing the recording capacity per disk. However, an increase in recording density without changing the diameter of the disk leads to narrower track pitch; accordingly, the technical problem is how accurate the head device for reading and writing data on record tracks is positioned, and a head actuator with good positioning accuracy is desired.
Hitherto, in order to position a head with high accuracy in a magnetic disk device, generally, trials have been made to enhance rigidity of movable portions such as an actuator arm and raise the main resonance point frequency in in-plane directions. However, improvement of resonance point has a limit, and even if the in-plane resonance point of the movable portion can be raised, there is still the problem that vibration is generated due to spring characteristics of a bearing supporting the movable portion, resulting in a reduction of positioning accuracy.
As means for solving the above problem, a so-called two-stage actuator in which a second actuator for following the tracks, namely, a tracking actuator is mounted on the tip of an arm of a head actuator, has been proposed. The tracking actuator is for minutely moving the head provided at a tip portion of an arm, independently from motions of the head actuator, to achieve tracking of the head.
A two-stage actuator in which accurate positioning of the head is accomplished by utilizing laminate type piezoelectric elements as the tracking actuator for the two-stage actuator has been proposed. For example, two laminate type piezoelectric elements are disposed on both sides of an actuator arm, and a voltage is impressed in such a direction that the piezoelectric element on one side is elongated while the piezoelectric element on the other side shrinks, whereon the head is rotated in the direction of the piezoelectric element shrinking under the impressed voltage.
However, in the two-stage actuator utilizing the laminate type piezoelectric element according to the prior art, depolarization of the piezoelectric element occurs due to impression of a voltage in the direction reverse to the polarization direction of the piezoelectric element, exposing of the piezoelectric element to a high-temperature atmosphere, aging or the like; as a result, displacement per unit voltage gradually diminishes. Therefore, there is the problem that when the system is used for a certain long time, a desired stroke cannot be obtained. Furthermore, the two-stage actuator according to the prior art utilizing the laminate type piezoelectric elements has the disadvantage that productivity of the laminate type piezoelectric elements is bad, and high accuracy or precision of outer sizes of the elements is required, leading to high cost.
Two-stage actuators utilizing a shearing type piezoelectric element in place of the laminate type piezoelectric element having the above-mentioned many problems have been proposed in Japanese Patent Laid-open No. Hei 10-293979 and Japanese Patent Laid-open No. Hei 11-31368. A minute moving mechanism for head disclosed in the Japanese Patent Laid-open No. Hei 11-31368 has a three-layer structure in which two shearing type piezoelectric elements having different polarization directions are mounted on an electrode formed at the tip of a head arm, and a head suspension is mounted thereon through a movable member therebetween.
Therefore, this structure has a larger thickness from the head arm to the suspension, as compared with the conventional structure in which the suspension is fitted to the head arm with only a spacer. Thus, the two-stage actuator with the three-layer structure is unsuitable for reduction in thickness of the head actuator. Further, the increased thickness leads to greater distance between disk surfaces, whereby the number of disks mountable in the disk device is decreased, and storage capacity is also decreased as compared with the conventional disk device having the same height.
A minute moving mechanism for head which solves the above-mentioned problems has been proposed by the present applicant. In the invention of the previous application, an actuator base bent in a crank shape is fixed to a tip portion of an actuator arm. A base electrode, a shearing type piezoelectric element, a movable electrode and a movable plate are laminated and fixed on the actuator base, and a suspension is fixed to the movable plate. With the actuator base bent in a crank shape, the top surface of the actuator base and the top surface of the movable plate can be flush with each other, so that the minute moving mechanism for head utilizing the shearing type piezoelectric element can be made thinner.
In the invention of the previous application, electrical conduction must be provided between the shearing type piezoelectric element and the base electrode and the movable electrode, and, therefore, the base electrode and the movable electrode have been fixed to the piezoelectric element by use of a conductive adhesive. On the other hand, electrical insulation must be kept between the actuator base and the base electrode and between the movable electrode and the movable plate, and, therefore, fixation between the actuator base and the base electrode and between the movable electrode and the movable plate has been made by use of an ordinary insulating adhesive.
In the method of producing a micro-actuator using a conductive adhesive and an ordinary insulating adhesive according to the invention of the previous application, it is needed to selectively use the two types of adhesive, which leads to complicated production steps. Further, use of a conductive adhesive may easily be attended by generation of a short-circuit due to a mistake in production process. Thus, it is contemplated to contrive use of a single adhesive by using an insulating adhesive also for adhesion of the electrode and the piezoelectric element. However, where an adhesive is merely applied and cured, an adhesive layer thin enough to obtain electrical connection cannot be formed.
Therefore, a method of applying a pressure at the time of curing the adhesive is contemplated. However, where pressure is merely applied upon each lamination of component parts through an adhesive, the thickness of the adhesive layer on application of pressure is added to the thickness of component parts, so that variations of the thickness of the component parts will come out as large variations of the total thickness. Further, where electrical connection is envisioned, two shearing type piezoelectric elements are mounted on the base electrode, and the difference between the thicknesses of left and right piezoelectric elements may cause variations of the adhesive layer even if they are pressed in the same manner.